Abstract: An improved system and method of monitoring the operation of a respiratory or surgical humidifier system. A controllable load can be connected across output terminals to a tube heating element to test the operation of a tube heating element supply circuit. A range of tests may be performed by controlling the state of one or more components and monitoring one or more sensors. Tests may be performed either during or not during therapy and either with a gas supply tube connected or not. Transient currents may also be detected and the power supply to a tube heating element can be stopped where transient currents outside a permitted range are detected.